Cob clay sand mixes

[millerputnam]

I am new on this forum and I'm looking forward to the help I can get with a cob oven. I think I have ALL the books... at least, the three that seemed most relevant. Where I'm lacking guidance is locating the proper clay in the soil. What we have predominantly here in South Carolina is what we call "red mud'. I've done the flexible test rolling it into a snake. I've made test bricks with one, two, and three parts sand. But being new to this process, I am not sure this red clay is suitable. I'd hate to go to the effort to make a cob oven with the local clay only to find it's not the best to use. Anyone have any suggestions?

miller

[lwalper]

Re: Cob clay sand mixes

Not sure about the chemical composition of your clay, but it's probably suitable. If it will roll into a rope, that's pretty sticky stuff. What I've seen demonstrated as appropriate "clay" for cob ovens would be a mixture of sand and clay with enough sand in the mix to form a ball when squeezed in the hand. Then taking a golfball sized lump sort of mashed flat in your hand covering your entire palm, turn your open hand over and give it a few good shakes. If the clay/sand mixture is correct it should take 5 to 7 shakes to fall free. That's no hard and fast number, but would probably get you in the ballpark for proper consistency and percentages of clay/sand. If it drops off too quickly, add clay; if it sticks too long, add a bit of sand. Remember, this is not cement with a working time limit. You've got time to play around with the stuff.

That mix is then thoroughly blended with chopped straw for your cob mixture. I've seen this done on a canvas placed on the ground, treading with bare feet and turning the mixture with an edge of the canvas -- just roll the mix into the center and tread some more.